1. Field of the Invention
This invention relates to a method of correcting a radial force variation (hereinafter abbreviated as RFV) of a tire and an apparatus therefor, which can effectively and advantageously improve RFV among uniformity characteristics of a pneumatic radial tire using an organic fiber cord as a carcass ply cord, particularly a small-size tire typified by a radial tire for a passenger car.
2. Description of Related Art
It is well-known that the uniformity of the tire, particularly the radial tire is an important property controlling the good or bad tire performances. Among the uniformity characteristics, RFV affects the performances of almost all radial tires and particularly has a remarkable influence upon steering stability and ride comfort against vibrations of the radial tire for passenger car, uneven wear of the tread portion and the like. Therefore, an acceptable upper limit of RFV is defined every kind of tire.
The measurement and characteristics of RFV will simply be described below.
When the tire attached to a uniformity testing machine is added with a load and rotated at a state of fixing a loading radius, reaction force during the one revolution of the tire always varies large or small, and a full amplitude of such a varying quantity is called as RFV. When the reaction force of the tire is plotted on an ordinate and a rotating distance in the one revolution of the tire is plotted on an abscissa, the variation of the reaction force measured by the testing machine can be taken out as a waveform wherein a primary component is generally an approximately sine wave form and secondary or more components are superimposed thereon. An example of the primary component wave form in RFV is shown by a solid line in FIG. 5.
The RFV of the tire mounted onto a vehicle is a varying quantity of a reaction force produced from a road surface in a radial direction per one revolution of the tire during the running under loading. The variation of the reaction force is a force applied to the vehicle, so that the tire having a large RFV value brings about disadvantages that the ride comfort against vibration in the vehicle is degraded and uneven wear is sometimes generated in the tread portion and particularly the steering stability of the vehicle during the running at a high speed is considerably damaged, and the like.
For this end, it is required to restrict the RFV of the tire to a range not causing these disadvantages. In the tires attaching importance to the uniformity characteristics, particularly radial tires for passenger car, therefore, each of all tire products after the vulcanization is mounted onto a given rim and inflated under a given internal pressure and subjected to an inspection sorting acceptance or rejection of uniformity, during which tires exceeding a given RFV value are removed off from a forwarding line as a reject product.
And also, the uniformity of the tire includes vertical vibration and lateral vibration based on the change of the size in addition to the aforementioned force variation. Among these vibrations, the vertical vibration in the radial direction of the tire particularly affects the tire performances. An absolute value of the vertical vibration in the radial direction is called as radial runout (hereinafter abbreviated as RR). In general, the RR is said to have a close relation to the RFV.
Therefore, the RR and RFV are simultaneously measured in the inspection sorting the acceptance or rejection of the uniformity. The tires rejected by the RFV are marked at a position showing a maximum RR value and removed off from the inspection line and subjected to correction of RFV. In this correcting method, the RFV-rejected tire is mounted onto a given rim and inflated under a given internal pressure, and thereafter a surface of a tread rubber marked is subjected to buffing through a grinder to cut off only a part of the tread rubber by a gauge in accordance with the RFV value, whereby the value of RFV is reduced.
And also, the RR waveform and the RFV waveform do not necessarily correspond to each other at both peak positions. There is also a method wherein the RFV correction through the above buffing is directly carried out to a part of the tread portion centering a position indicating a maximum RFV value in a primary component taken out from the RFV waveform along a circumference thereof.
However, even if the reject tires can be saved by these methods of correcting the RFV value to an adequate range through the buffing, it is substantially impossible to completely restore the appearance of the tread portion subjected to the buffing and hence the lowering of the tire appearance value is undeniable. Furthermore, there can not be overlooked a problem that rubber dust generated by buffing the tread rubber damages the working environment.
For this end, it is attempted to take shaping factor and curing factor among factors of the RFV and minimize the RFV value, i.e. offset these shaping factor and curing factor to each other to raise a value of minimum value A and lower a value of a maximum value B in RFV shown by a primary component waveform of RFV in FIG. 5 and hence reduce the value of RFV. However, such an attempt is critical in the correcting quantity and is insufficient in the improvement of RFV.
In this connection, U.S. Pat. No. 5,616,859 proposes that RR is adopted as a target for correcting the uniformity and a part of cords in at least one ply existing in a sidewall portion is subjected to permanent deformation so as to make the RR below an acceptable value. Such a permanent deformation is carried out by restraining a part of the tire located other than a position to be corrected under an inflation of the tire at a previously set pressure and restricting the stretching of the cord in such a restrained portion.
According to the method described in the above U.S. patent, the correction of RFV can be carried out without damaging the tire appearance if positions indicating the maximum value and the minimum value correspond to each other between RR and RFV. However, since a part of the carcass ply cord is permanently deformed, it is required to apply a considerably high tension to the carcass ply cord and also the internal pressure of the tire should be rendered into a considerably higher value, and hence there is a fear of breaking the tire during the formation of the permanent deformation in the cord.
In order to solve the problem of RFV correction disclosed in the above U.S. patent, the inventors have proposed a method wherein RFV itself is adopted instead of RR in the RFV-rejected tires and the maximum value of RFV is decreased and the minimum value of RFV is increased and as a result RFV is corrected without being subjected to a cutting work such as buffing or the like and inflating under a considerably high internal pressure in U.S. patent application Ser. No. 09/492,853.
According to this method, after RFV of a tire inflated under a given internal pressure is measured, a first distinguishing mark is put to a first tire portion indicating a minimum value of RFV and a second distinguishing mark is put to a second tire portion indicating a maximum value of RFV, and different thermal hysteresis are applied and a state of applying a given internal pressure to the tire is kept during the cooling in the thermal hysteresis. It has been confirmed that this method can develop an excellent effect.
In the RFV correction of the above US patent and US patent application, however, the correcting quantities of RR and RFV are confined to slight values, respectively, when a cord being small in the elongation and very small in the thermal shrinkage such as rayon cord or the like is used in the carcass ply of the tire. In this point, it is undeniable that the RFV correction is insufficient. On the other hand, when nylon cord or polyester cord being large in the thermal shrinkage is used in the tire, the RFV value tends to return to a value before the correction accompanied with the rise of the temperature under the occurrence of strain during the running of the tire under loading.